The subject invention is directed toward the art of internal combustion engines and, more particularly, to an improved rocker arm assembly and method of making the same.
The invention is especially useful in high performance, high RPM engines of the type used in racing and will be described with particular reference thereto; however, it should be appreciated that the invention is useful in a variety of types of internal combustion engines used for many purposes.
High performance engines used in racing are subjected to extreme operating conditions including extended periods at top RPM's, sudden and rapid changes in loading, and the like. As is apparent, this results in extreme forces and changes in the forces applied to various engine components. Coupled with the inherent demands placed on the components has been the desire to increase performance and efficiency by reducing the size and weight of the individual components.
One component which has been especially vulnerable to failure has been the rocker arm members used to transmit movement from the cams to the engine intake and exhaust valves. These members are subjected to extremes in both dynamic and impact loading. A particularly vulnerable point has been the contact area between the push rod and the rocker arm, generally referred to as the push rod seat. Merely by way of example, when operated at peak RPM's, the push rod seat area may be subject to impact force variations of from 0 to 1500 pounds at 5000 times per minute. In addition, these forces may be applied over a 20 to 30 degree arcuate variation in direction relative to the seat area.
Attempts have been made to reduce the weight of rocker arms by forming their main body from light weight metal alloys, such as aluminum alloys, with the push rod seat area comprised of a hardened steel insert member interference fitted to the aluminum body. These attempts have not been especially successful. The constantly changing loads and their changing directions of application have caused fatigue in the aluminum body about the steel insert. As a result, the inserts have tended to loosen and "walk-out" of the rocker arm body.